1. Technical Field of the Invention
This invention relates generally to wireless communication system and more particularly to power amplifiers of wireless communication devices.
2. Description of Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies then. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
As is also known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier, when enabled, amplifies the RF signals prior to transmission via an antenna.
The output section of the power amplifier includes a power down P-channel transistor operably coupled to a gate of an N-channel cascode transistor, a bias circuit, and an N-channel input transistor having its gate coupled to the bias circuit and to an AC coupling capacitor to receive the RF signals. When the power amplifier is enabled, the power down P-channel transistor is on, thus enabling the N-channel cascode transistor. With the N-channel cascode transistor enabled, the N-channel input transistor amplifies the RF signals and, in conjunction with the N-channel cascode transistor, outputs the amplified RF signals.
When the power amplifier is not enabled, the power down P-channel transistor is off, which pulls the gate of the N-channel cascode transistor to ground. With a 3.3 volt supply, the gate-drain voltage and the drain-source voltage of the N-channel cascode transistor is 3.3 volts, which stresses the gate oxide layer of the transistor. Since the power amplifier is primarily in the power down mode, the gate oxide layer of the N-channel cascode transistor is stressed at 3.3 volts, which, over time, causes failures of the N-channel cascode transistor.
Therefore, a need exists for a power amplifier having low gate oxide stress.